Brine feed specifications

Ihble 13. Typical brine feed specifications for diaphragm cells... 

The brine feed to the electroly2ers of all the processes is usually acidified with hydrochloric acid to reduce oxygen and chlorate formation in the anolyte. Table 14 gives the specifications of the feed brines requited for the membrane and diaphragm cell process to reali2e optimal performance. 

Most commonly, diaphragm cells are supplied with well brine on a once-through basis. The treated well brine flows to the treated brine storage tanks, which usually have 12-h capacity. From there the brine is fed to the cell room. The flow to each individual electrolyzer is controlled by a rotameter. If the flow of brine to the cells is suddenly disrupted by failure of the brine feed pump, the rectifiers automatically shut down since an inadequate supply of brine to the cells is potentially unsafe. The specifications for brine for diaphragm cells are given in Table 13. 

Table 14. Typical Specifications for Feed Brine to Electrolyzers...

Elsewhere in this book, White and Sandel [7] discuss the integration of chlorine and ethylene dichloride (EDC) processes. The oxygen content of the chlorine fed to an EDC unit must be kept within the process specification. This can be achieved by liquefying at least part of the chlorine in order to reject non-condensables or by acidifying the brine fed to the cells. Oxygen results from the anodic oxidation of hydroxide ions free acid in the feed brine will neutralise those ions and so reduce the amount of oxygen formed. 

Viton hoses were instead selected for the feed brine to the electrolysis cells. These are chemically resistant to chlorine-containing brine. There are several specifications of Viton hose available. For working with brine in an electrolysis environment, special attention had to be given to rupture resistance of the hoses with respect to operator safety. 

The decision to retrofit or install new filters is influenced by plant-specific factors such as the condition of the existing filters, compatibility of the filters to retrofit, availability of space within the plant, etc. Bayer s analysis of the costs and benefits of the retrofit compared to the investment in new filters did not show a clear favourite either decision could be justified. With regard to the rubber lining, gaskets, hydraulic system etc., the Kellys were in very good condition. Because of this, the decision was made to retrofit two of the filters and to use them in a separate brine circuit for the new electrolysers, with a second circuit feeding the amalgam plant. 

The second typical technology applied for d. of water is -> electrodialysis. After appropriate pretreatment (as above), the feed solution is pumped through the unit of one or more stacks in series or parallel. The concentrated and depleted process streams leaving the last stack are recycled, or finally collected in storage tanks. The plants operate unidirectionally, as explained, or in reverse polarity mode, i.e., the current polarity is changed at specific time intervals (minutes to hours), and the hydraulic flow streams are reversed simultaneously, thus preventing the precipitation in the brine cells. 

The second stage of the hybrid-process is essentially a crystallization capable of crystallizing all solids dissolved in the feed resp. the brine of the RO stage. Parts of these substances tend to heavy scaling and, therefore, the choice of the crystallization equipment is limited. A proven solution is, of course, the agitated thin film evaporator. However, this type of equipment must lead to high specific treatment costs for two reasons ... 

Case Membrane type / membrane array Feed temp. (°C) Feed pressure (bar g) Reject/ brine pressure (bar g) Product flow (mVh) PWR= (%) Avg. flux. (l/m /h) Product IDS (mg/l) RO pump power (kW), (1) Energy recov. (kW), (2) Energy recov. (%) RO motor energy (kW), (1) (2) Specific energy (kWh/ m")... 

Notes Feed water flow rate=45 m /h. Feed water TDS = 34,252 mg/1. Reject/brine osmotic pressure range=38 (A) - 51 (H) bar. RO pump // 80%. RO pump motor rj=90%. Energy recovery mrbine >y=88%. Calculated specific energy for the SWRO unit only. Source Singh [8]. 

Energy consumption is measured in kilowatt-hours per ton of product, the product being either chlorine or caustic. Most operators and technology suppliers choose caustic as the basis for measurement. This choice reflects the practical difficulties of measuring chlorine production accurately and taking into account system losses that end up principally as hypochlorite or HCl. Another comphcation is the dependence of anolyte current efficiency on the amount of acid or alkali present in the feed brine (Section 7.5.6.1). The caustic current efficiency, for all practical purposes, depends only on the membrane efficiency. It becomes more convenient and usually more accurate to measure the production of caustic. One need only measure the amount of solution produced and analyze its caustic content. Again for convenience and accuracy, and assuming the use of membrane cells, it is best to measure the output of cell liquor. This separates the electrolyzer and evaporator test runs. These measurements make it possible to calculate the anode current efficiency from analytical data and hence, to calculate chlorine production and specific power consumption. 

The feed brine concentration normally is fairly closely controlled in the brine plant. A typical operating specification is 300 gpl NaCl. Higher concentrations can lead to salting out in the electrolyzers as the system cools when offload. The proper combination of feed and exit brine concentrations gives satisfactory utilization of the salt. 

Hydrochloric Acid Specification. The specification in Table 13.2 is generally applicable for electrolyzer feed brine acidification, depleted brine pH control, and ion exchanger regeneration. The contaminant levels relate to normally available commercial concentrations of 32-36% HCl. Hydrochloric acid used for brine acidification should be sufficiently dilute to prevent any risk of salting out. This aspect is covered in Section 7.5.6.1. 

As with the electrode coatings, the membranes only operate to specification under tightly controlled conditions. In particular, the brine and catholyte feeds must be highly purified. Sulphate and group II metals lead to precipitation within the membrane while heavy metals (and iron from corrosion) complex the carboxylate groups and cause an increase in membrane resistance. 

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